Exposures of olivine-rich rocks in the vicinity of Ares Vallis:Implications for Noachian and Hesperian volcanism

J. H. Wilson1 and J. F. Mustard1

Received 15 May 2012; revised 12 February 2013; accepted 28 February 2013; published 9 May 2013.

[1] The igneous evolution of Mars is well represented in stratigraphic settings thattransition across major time stratigraphic boundaries. Here we analyze in detail themorphology and composition, determined through visible–near-infrared spectroscopy, ofigneous volcanic terrains in Ares Vallis, Mars. Upland plateau units with crater-fillingvolcanic plains are of Noachian age; smooth units to the west and north of the mouth ofAres Vallis have been mapped as Hesperian in age. The age and origin of the units thatcomprise the floor of Ares Vallis, connecting the upland plateau units and the smooth unitsto the west and north of the mouth of Ares Vallis, is less certain due to the small areaavailable for crater counting. The mafic mineral compositions of these units are wellresolved with OMEGA (Observatoire pour la Mineralogie, l’Eau, les Glaces et l’Activité)and CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) data. The datashow that the Noachian volcanic units are distinctly olivine-rich, while the plains units tothe west and the floor of Ares Vallis volcanics show diagnostic absorptions of olivine-pyroxene, typical of Hesperian volcanics elsewhere on Mars. Based on these findings, wepropose there has been a change in the temperature and/or degree of partial melting in themantle over time for this region of Mars.

Citation: Wilson, J. H., and J. F. Mustard (2013), Exposures of olivine-rich rocks in the vicinity of Ares Vallis:Implications for Noachian and Hesperian volcanism, J. Geophys. Res. Planets, 118, 916–929, doi:10.1002/jgre.20067.

1. Introduction

[2] Understanding the history of surface volcanism onMars is important in piecing together the igneous evolutionof the planet and the role that volcanism may have had inthe evolution of the atmosphere and subsurface [Craddockand Greeley, 2009; Carr and Head, 2010]. Volcanism iswell recognized as the dominant form of resurfacing in theHesperian era where the northern lowlands were resurfaced[Head et al., 2002], extensive ridged plains units of thesouthern highlands were formed (e.g., Syrtis Major)[Hiesinger and Head, 2004], and the ridged plains surround-ing and forming the upper sections of Valles Marineris wereemplaced [e.g., McEwen et al., 1999]. Hesperian volcanicunits have been extensively analyzed with remotely senseddata [e.g., Mustard et al., 1997; Christensen et al., 2001;Rogers and Christensen, 2007; Rogers et al., 2007; Hahnet al., 2007; Baratoux et al., 2013], which shows these rocksare dominantly basaltic in composition. Mineralogy deter-mined from visible near-infrared (VNIR) and thermal

infrared (TIR) data shows an olivine basalt compositiondominated by high calcium pyroxene (HCP) and plagioclase[Poulet et al., 2009b; Rogers and Christensen, 2007; Rogerset al., 2007]. In contrast, although Noachian volcanism iswidely understood to be an important process [e.g., Phillipset al., 2001; Jakosky and Phillips, 2011; Spohn et al., 2001;Werner, 2009], clearly defined Noachian-aged volcanic plainsthat can be characterized compositionally with remotelysensed data are comparatively rare.[3] Nearly half of the Martian surface is thought to be of

definitive volcanic origin [e.g., Greeley and Spudis, 1978;Mouginis-Mark et al., 1992; Greeley et al., 2000; Headet al., 2001]. In general, the Noachian era saw a peak involcanism, which gradually decreased over time [Headet al., 2001] and is consistent with the idea that Tharsiswas mainly in place by the end of the Noachian [Phillipset al., 2001; Jakosky and Phillips, 2011; Spohn et al., 2001].Volcanism in the Noachian era was likely concentrated inthe Tharsis region [e.g., Phillips et al., 2001; Carr and Head,2010], where some of the volcanics are exposed in the layeredwalls of VallesMarineris [e.g.,McEwen et al., 1999;Hamiltonet al., 2003; Christensen et al., 2003, 2005; Edwards et al,2008; Koeppen and Hamilton, 2008; Flahaut et al., 2012]. Ithas been demonstrated with remotely sensed data [Bandfieldet al., 2000; Bandfield, 2002; Mustard et al., 2005; Rogersand Christensen, 2007; Rogers et al., 2007; Koeppen andHamilton, 2008; Poulet et al., 2009a, 2009b; Rogers andFergason, 2011] that the mineralogy of the oldest Martiansurfaces and studies of Martian meteorites [McSween, 1994;Nyquist et al., 2001] are suggestive of a crust that is enriched

Additional supporting information may be found in the online versionof this article.

1Department of Geological Sciences, Brown University, Providence,Rhode Island, USA.

Corresponding author: J. H. Wilson, Department of GeologicalSciences, Brown University, Box 1846, Providence, RI, 02912, USA.(janette_wilson@brown.edu)

©2013. American Geophysical Union. All Rights Reserved.2169-9097/13/10.1002/jgre.20067

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JOURNAL OF GEOPHYSICAL RESEARCH: PLANETS, VOL. 118, 916–929, doi:10.1002/jgre.20067, 2013

in mafic minerals. Results from the thermal emission spec-trometer (TES) data [Bandfield et al., 2000; Bandfield, 2002]showed that the surface of Mars can be broadly characterizedinto two surface types of either basalt or basaltic andesite toandesite, where the mineralogy consists mostly of highcalcium pyroxene (HCP) and plagioclase feldspar with nolow calcium pyroxene (LCP) above the detection limits(5%–10%). Additionally, olivine has been detected globallywith various instruments including TES [e.g., Rogers et al.,2005; Hoefen et al., 2003; Bandfield, 2002; Hamilton andChristensen, 2005; Koeppen and Hamilton, 2008], THEMIS[e.g., Rogers et al., 2005; Hamilton and Christensen, 2005],the Observatoire pour la Mineralogie, l’Eau, les Glaceset l’Activité (OMEGA) [e.g., Mustard et al., 2005; Bibringet al., 2005; Poulet et al., 2007], and the MER rovers in situ[e.g., Christensen et al., 2004; McSween et al., 2004;Morriset al., 2004]. From the VNIR analyses of later missions,although high-calcium pyroxene is present in all age terrains,olivine and low-calcium pyroxene are the most commonlydetected minerals in older terrains and low calcium pyroxenecontent decreases with surface age [e.g., Mustard et al.,2005; Bibring et al., 2005, 2006; Poulet et al., 2007,2009b; Flahaut et al., 2012]. Skok et al. [2012] agree that,in Noachian terrains, olivine and low-calcium pyroxene tendto occur in distinct, highly segregated concentrations ofeither pyroxene or olivine, which is different than the natureof Hesperian material that shows more of a lithology ofolivine and pyroxene; however, studies using TES andTHEMIS data support crustal heterogeneity [e.g., Rogerset al., 2005; Rogers and Fergason, 2011].[4] The continuation of the volcanic record that occurs near

Valles Marineris into the Hesperian era is also preserved at thetop-most section of the walls [e.g., Greeley and Guest, 1987;McEwen et al., 1999; Flahaut et al., 2012]. This type ofHesperian volcanism, also known as the ridged plains[Greeley and Spudis, 1981; Tanaka, 1986], has been estimatedto have resurfaced about 30% of the Martian surface [Headet al., 2002; Carr and Head, 2010]. The majority of thesurface expression of Hesperian volcanics occurs in the north-ern lowlands [Greeley and Spudis, 1978; Tanaka et al., 1988;Mouginis-Mark et al., 1992; Greeley et al., 2000] is no morethan 1 km thick and is underlain by a cratered, Noachian-aged surface [Frey et al., 2001;Head et al., 2001] that exhibitsthe typical ancient-altered crustal mineralogical signatures[Carter et al., 2010]. The northern lowland volcanics havebeen shown to be continuous with ridged plains farther south[Head et al., 2002; Carr and Head, 2010]. Additionally, ithas been noted that the Hesperian ridged plains within theheavily cratered terrain are likely volcanic [Head et al.,2006; Carr and Head, 2010]. Hesperian volcanics, in general,have orbital signatures consistent with a mixture of olivine,low-calcium pyroxene, and high-calcium pyroxene with thelatter being the more abundant of the pyroxenes [Mustardet al., 2005; Bibring et al., 2005, 2006; Poulet et al., 2007,2009b; Salvatore et al., 2010; Skok et al., 2012].[5] While determining the precise timing of volcanism in

ancient terrains is often troublesome for areas that are notnear volcanic centers, the composition of Noachian-agedvolcanic terrains warrants further study. The crateredNoachian terrain just to the east of Aram Chaos [Tanakaet al., 2005] is of interest for three main reasons: (1) a signif-icant amount of volcanism has been proposed to have taken

place on Mars in the Noachian [cf. Greeley and Spudis,1981], some of which is preserved throughout theNoachian-aged plains studied herein; (2) change in igneouscomposition can be assessed since well-exposed Noachian-aged volcanics are in close proximity to igneous terrain thatis mapped as Hesperian; and (3) exposure in this region isgood as compared to other dusty Noachian terrains. Thestudy area, in particular, exhibits resurfacing in the form offilled or ghost craters bearing some of the most olivine-richcompositional signatures detected by the TES [e.g., Koeppenand Hamilton, 2008]. Further characterization of theseresurfaced plains units may give us more insight into themanner and location of Noachian-aged volcanism. Moreover,units beyond the mouth of Ares Vallis and into the northernplains are clearly Hesperian in age; thus, the stratigraphy fromthe Noachian highlands to the Hesperian lowlands stretchesacross a transitional temporal boundary, providing insight intothe igneous evolution of the region.[6] In this study, we characterize the composition and mor-

phologies of units in the study area, present strong evidencefor a compositional change from more to less olivine-rich inthe Ares Vallis region on Mars, and hypothesize why thischange occurred. Areas with igneous signatures in and aroundAres Vallis can be detected with the high-resolution CompactReconnaissance Imaging Spectrometer for Mars (CRISM)[Murchie et al., 2007] and the OMEGA instrument on MarsExpress [Bibring et al., 2004]. OMEGA is especially keyin allowing the broad characterization of the widespreadNoachian plains units that stretch between Ares Vallis andMeridiani Planum. Their morphological and geologicalcontext are assessed with the Context Camera (CTX) [Malinet al., 2007], the High-Resolution Imaging Science Experi-ment (HiRISE) [McEwen et al., 2007], as well as otherimaging data, allowing for a more detailed geomorphiccharacterization of the deposits than has been performedpreviously. In addition, Mars Orbiter Laser Altimeter(MOLA) topography data [Smith et al., 2001] aids in thecharacterization of stratigraphy of units and excavationdepth calculations.

2. Background

2.1. Setting

[7] Ares Vallis is an approximately 1,500 km long, 2 kmdeep channel that originates in Iani Chaos near 17.5�Wlongitude, 5�S latitude (Figure 1). The channel is up to300 kmwide, near the mouth, and 21 kmwide at its narrowest.The entire surface of the region, including the shape of AresVallis, has been structurally controlled by the Chryse andAram Chaos Basins (see section 5.1. and Figure 1a) [Schultzet al., 1982]. The channel is surrounded by older highlandplains to the east that appear to be resurfaced. Only onedetailed morphological and mineralogical study has beenperformed in this area to date and is discussed further insection 2.3 [Rogers et al., 2005].

2.2. Ages of Units

[8] Crater counting of valleys and channels is particularlydifficult because the area for counting is small and waterand wind can modify a cratered channel floor, causing it toappear pitted (pits can easily be confused as small craters).The eroded craters appear to be smaller than their noneroded

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counterparts and, therefore, return a younger age than that ofthe actual formation. In addition, any subsequent deposits lainin a channel floor can also cover up craters, having the sameeffect. Tanaka et al. [2005] dated these surfaces and deter-mined the age of the main channel of Ares Vallis to be middleof the Late Hesperian (Figure 1b), whereas the date of lastfluvial activity for the large tributary to the west, which flowednorth eastward around Aram Chaos, has been dated by War-ner et al. [2010] to be Amazonian in age. Some even

propose [e.g.,Marchenko et al., 1998] that Ares Vallis is EarlyHesperian in age, whereas the Tiu Valles system that crosscutsAres Vallis at its mouth is Late Hesperian to Early Amazonianin age. Regardless, it is evident that Ares Vallis has experi-enced the last major episodes of fluvial and/or volcanicresurfacing between the Hesperian (main channel) and EarlyAmazonian (tributary).[9] Tanaka et al. [2005] map four units of interest to this

study in two key regions (Figures 1b and 2). The first region,the Noachian plains surrounding Ares Vallis, contains unitsNl and Nn. Nl is early to middle Noachian in age anddescribed as a combination of reworked, volcanic, sedimen-tary, and primordial crustal materials. Nn is middle to lateNoachian in age and described as a combination of reworked,volcanic, and sedimentary resurfaced materials. The secondregion, the Hesperian units in the floor of Ares Vallis, iscomposed of units HCa and HCc4. HCa is late Hesperian inage and interpreted to be an indurated cap of possible igneousorigin, while HCc4, also late Hesperian, is interpreted as adebris flow or compacted sedimentary material. A time-related context is clearly present for separating spectraldetections on the floor of Ares Vallis from the plains to theeast. This framework, on the basis of the mapping of Tanakaet al. [2005], establishes that the plateaus to the east of AresVallis are Noachian in age, while the floor of Ares Vallisand the outflow channel region to the north and west areHesperian. While this provides the relative age of the sur-faces exposed in the region, it is not clear if the flat floorof Ares Vallis is a depositional or erosional unit. It may bea postfluvial, volcanic deposit, or it may be a consistenterosional unit from outflow channel formation. In addition,the map scale of Tanaka et al. [2005] (1:15M) is relativelycoarse compared to some of the features of interestpresented: spectrally unique, small-scale exposures on thechannel floor may differ in age from the larger map unit thatcontains them.

Figure 1. (a) MOLA elevation map of the circum-Chryseregion overlain on a shaded relief map. Ares Vallis can beseen with two distinct bends in its direction, labeled A andB. These bends are controlled by the deep-seated faultsand fractures caused by the Chryse and Aram Basin impacts[Schultz et al., 1982]. Some of the many identifiable basinrings are shown in black. (b) View of the region outlinedby a white box in Figure 1a with the age units of Tanakaet al. [2005] overlain on MOLA shaded relief. For all unitdescriptions, please refer to Tanaka et al. [2005]. Selectunits of interest are highlighted in Figure 2 and describedin section 2.2.

Figure 2. Select units, described in section 2.2, are shownoverlain on MOLA shaded relief. Note the flat-floored,infilled craters, some of which are breached (arrow). Taytaycrater, marked by the X is a window into the resurfacedmaterial filling these craters. Black outlines indicate imagefootprints where spectral data were acquired and are includedfor reference to Figures 3 and 8.

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2.3. Previous Studies

[10] Ares Vallis is one of many outflow channels on Marsgenerally thought to have formed by the catastrophicoutflow of subsurface water [e.g., Carr, 1979; Baker,1982; Baker et al., 1992; Clifford et al., 1993]. A numberof other processes for channel formation including lava flow,aeolian processes, flow of water ice, flow of carbon dioxide,emplacement of debris flows, and crustal extension havealso been proposed [Leverington, 2011, and referencestherein]. Catastrophic outflow of subsurface water is themost accepted view, yet the general observations show thatalteration minerals are lacking in and around most outflowchannels [e.g., Bibring et al., 2006; Mangold et al., 2007,2008]. As mentioned previously, olivine is one of the morereadily detected mineral signatures on Mars and particularlyin the Ares Vallis region. The presence of diagnostic signa-tures of igneous compositions in Ares Vallis (and otherchannels), as opposed to alteration minerals may indicate a his-tory where aqueous activity was not the sole process at work.[11] Generally, a few studies have interpreted the signifi-

cance of olivine on large scales, which included detectionsin the Ares Vallis region. Koeppen and Hamilton [2008]mapped olivine globally using TES data and grouped thedeposits by their magnesium content, expressed in Fonumber, noting that the most common Fo numbers of theirend-members were Fo68 and Fo53; the most commondeposits were observed to occur in the floors of outflowchannels and smooth-floored craters. Furthermore, Edwardset al. [2008] imply that regional olivine-rich deposits associ-ated with outflow channels may reflect extensive flooding byprimitive lavas during the Noachian, which were later exposedby outflow channel erosional events.[12] Volcanism has been thought to have caused resurfacing

in the northern portion of Ares Vallis, consistent with thevolcanic Hesperian-aged northern plains units [e.g., Robinsonet al., 1996]. Salvatore et al. [2010] and Ody et al. [2011,2012] verify with CRISM and OMEGA, respectively, thatolivine-rich areas exist in the Hesperian-aged northern plains,at the mouth of Ares Vallis and beyond. This igneous signa-ture could be attributable to the Hesperian ridged plainsvolcanism outlined by Head et al. [2002]. However, it isnot clear whether this potential volcanism is related to theolivine seen in the southern portions of the channel.[13] In particular, Rogers et al. [2005] performed a detailed

study of a larger portion of the region studied here using TESand THEMIS data, revealing that there are many olivine-richareas, including solid rock outcrops, which they attribute toone or more regional bedrock units that could be intrusive orextrusive in nature. Rogers et al. [2005] point out three mainunits in the region. The unit on the floor of Ares Vallis hasthe lowest albedo, but intermediate thermal inertia. Mineral as-semblage assessment fromTIR data estimates that this unit hasa mineralogical makeup of 30% feldspar, 25% pyroxene [80%high calcium pyroxene (HCP) and 20% low calcium pyroxene(LCP)], 25% high silica materials, and 20% other. The lowestthermal inertia unit corresponds to the layered channel wallsand the plains units to the east, consisting of 20% feldspar,15% pyroxene (80% HCP, 20% LCP), 35% high silicamaterials, and 30% other. Finally, the highest thermalinertia unit, which is a cliff-forming unit of fractured, in placerock, has a mineralogical composition of 45% pyroxene

(>90% HCP), 25% olivine, and 30% other. In particular, thisunit was reported to have a feldspar content below the detec-tion limit, which is a striking characteristic as compared tothe other units in the area. Rogers et al. [2005] report thatthe olivine-rich unit is similar in thermophysical character toother olivine-bearing terrains on Mars, except the mineralogyreflects more pyroxene and less plagioclase than the otherolivine-bearing terrains. Finally, Rogers et al. [2005] did notinclude a pigeonite spectrum in their deconvolutions, whichmay result in an overestimation of the HCP component.[14] Mars Express and the Mars Reconnaissance Orbiter

(MRO) have been returning complimentary spectral datawith OMEGA and CRISM and imagery that we use in thepresent study to consider these units in more detail, withthe intent of further constraining the geological significanceof the previously identified units and investigating thecomposition of Noachian-aged units.

3. Data Sets and Methods

3.1. Global Data Sets

[15] Global data and higher-resolution imagery were com-bined using a Geographic Information System (GIS) for thebulk of the analysis. Global data used include the ThermalEmission Imaging System (THEMIS) daytime infraredmosaic and Mars Orbiter Laser Altimeter (MOLA) [Smithet al., 2001] topographic data set, both acquired from theU.S. Geological Survey (USGS), at 256 and 128 pixels perdegree, respectively. Gridded elevation data from the globalMOLA map were used to create the terrain-correctedtopographic profiles shown in Figure 3. Age units used inthe GIS, from Tanaka et al. [2005], were acquired fromthe USGS online database. Select units from the northpolar map are shown in Figures 1b and 2.

3.2. Imagery

[16] High-Resolution Science Experiment (HiRISE), Con-text Camera (CTX), and High-Resolution Stereo Camera(HRSC) imagery were used to assess different scales of theregion’s morphology. HRSC has a spatial resolution of about10m/pixel [Neukum et al., 2004]; the specific images analyzedherein have a resolution between about 11 and 34m/pixel.CTX images the surface of Mars at approximately 6m/pixel[Malin et al., 2007]; HiRISE images the surface at a muchhigher spatial resolution than CTX, at 25–130 cm/pixel in grayscale and color (channels are in the red, NIR, and blue–greenranges) [McEwen et al., 2007]. Along with the global datasets, CTX and HRSC images along the entirety of Ares Valliswere compiled into the GIS. HRSC, CTX, andHiRISE imagesused in this study are outlined in Table 1. Figure 4–6

3.3. Spectral Data

[17] The Compact Reconnaissance Imaging Spectrometerfor Mars (CRISM) is a hyperspectral imager that collects datain two modes. The multispectral mapping data consist of 72wavelengths which are sampled at 200m/pixel; with fullspatial and spectral resolution, CRISM can collect data at15–19m/pixel over 544 channels from 362–3920 nm at6.55 nm/channel sampling [Murchie et al., 2007]. The mostcurrent publicly released calibration version of the data,TRR3 [Seelos et al., 2011], is used for images HRL00005B77and FRT00012CED (Table 2). Atmospheric effects and

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photometric corrections for these data are described by Seeloset al. [2011]. The atmospheric correction was applied bydividing the scaled volcano observation 61C4, using thePelkey 2-wavelength method for volcano scan scaling(2,011 nm/1,899 nm). The functions for photometric andatmospheric correction are included in the CAT softwaremenu provided by the CRISM team. Spectra were extractedfrom regions in the CRISM and OMEGA data that weredefined on the basis of spectral parameter maps [Pelkeyet al., 2007] using either 5 � 5 pixel averages or regions ofinterest drawn manually to cover the smaller hydrated min-eral units or irregular surface exposures. Residual artifactsafter calibration and atmospheric removal are suppressedby ratioing the spectra to a spectrally neutral region(e.g., Figures 7 and 8). Spectrally neutral or featureless sur-faces are recognized as showing no features in the summaryspectral parameters. For CRISM data, ratios are generated

using a neutral/featureless spectrum from the same columnsbecause of the nature of 2-D detectors. To ensure consistencyin ratios, both S and L data were ratioed prior to being joined.While the ubiquitous red slope in the visible wavelengthregion can lead to interpretation difficulties, that was notconsidered important for these data.[18] In addition, spectral data from OMEGA, on the Mars

Express spacecraft, are used in conjunction with CRISM.OMEGA is well suited for surveying the large plains area,since the coverage is more spatially extensive than that forthe CRISM targeted observations. OMEGA collected datain 352 spectral channels over three separate detectorsand covers the visible and near-infrared wavelength rangefrom 0.35 to 5.09 mm; the spectral resolutions of theVNIR (0.35–1.05 mm), SWIR (0.94–2.70 mm), and LWIR(2.65–5.2 mm) detectors are 7.5, 14, and 21 nm, respec-tively [Bibring et al., 2004]. Basic reduction of OMEGAdata from radiance to reflectance includes applying knowninstrumental corrections, conversion of DN (digital number)to radiance, division by the solar flux adjusted to theMars distance to reduce the data to I/F, and lastly dividingby the cosine of the incidence angle relative to the aeroid[Bibring et al., 2005]. Atmospheric removal proceeds aswith CRISM by assuming that the surface and atmosphericcontributions are multiplicative and that the atmosphericcontribution follows a power law variation with altitude[Bibring et al., 1989]. The model of atmospheric transmis-sion was derived from OMEGA observations from the topand bottom of Olympus Mons. This transmission spectrumis then scaled to each OMEGA observation by the strengthof the CO2 absorption and ratioed to the data to removeatmospheric contributions. The ratio technique is also usedfor examining OMEGA data; unlike CRISM, OMEGA is alinear array of spectrometers; thus, the featureless denomi-nator can be from anywhere in the image (e.g., Figure 4).All spectra included in this study are listed by extractionlocation in Table 2.[19] Reflectance spectra of the igneous minerals studied

herein (cf. Figure 9), olivine and pyroxene, possess reflectance

Table 1. IDs for Imagery Used, Listed by Figure Number

Sensor Image ID

Figures 2 and 3HRSC H0401_0000_ND3

H0901_0000_ND3H0912_0000_ND3H0923_0000_ND3

Figure 4HRSC H0923_0000_ND3CTX G02_019097_1864

Figure 6HiRISE ESP_013216_1870

Figure 7CTX B05_011515_1867

P07_003762_1861P08_004118_1857

Figure S1HiRISE anaglyph ESP_013216_1870

ESP_022143_1870

0

0.05

0.1

0.15

0.2

1

1.05

1.1

1.15

1.2

0.5 1 1.5 2 2.5

Cal

ibra

ted

Ref

lect

ance

Ratioed R

eflectance

Wavelength(µm)

Target Spectrum

Neutral SpectrumRatio

FRT00012CED, R: 2.3840, G: 1.8028, B: 1.1521

Figure 3. (a) CRISM image FRT00012CED. Floor materials where spectral detections are made arelight-toned. The black star (outlined in red) and white star represent the areas where the target and neutralspectra were extracted, respectively. (b) Spectral plot of floor materials showing a very broad 1 mm andsmaller 2 mm absorption feature indicative of a mixture of olivine and HCP. Please refer to the online colorversion of this figure for more detail.

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minima near 1 and 2 mm due to Fe2+ crystal field absorptions[Burns, 1970b, 1993] in the CRISM and OMEGA visible–near-infrared spectral ranges. Three overlapping absorptionsnear 1 mm due to crystal field transitions of Fe2+ in thedistorted M1 and M2 octahedral sites [Burns, 1993, and refer-ences therein; Sunshine and Pieters, 1998] define the presenceof olivine and shift to longer wavelengths with increasing Fecontent [e.g., Burns, 1970a; Adams, 1975; King and Ridley,1987; Burns, 1993; Sunshine and Pieters, 1998; Isaacsonet al., 2011], although the separate absorptions are rarelydiscernable in orbital spectra of olivines on Mars. Absorptions

near 1 and 2 mm define the presence of Fe2+ in the M1 andM2sites in pyroxene, respectively; band centers are around 0.9and 1.9 mm for LCP and 1.05 and 2.35 mm for HCP [Cloutisand Gaffey, 1991; Cloutis, 2002]. The band centers for pyrox-enes vary systematically [Burns, 1970b, 1993] from shorter tolonger wavelengths with increasing calcium and iron con-tent [e.g., Adams, 1974; Cloutis and Gaffey, 1991].[20] In Figure 4, the strength of the olivine detection for

OMEGA data was measured using a modified version ofthe OLINDEX2 parameter from Salvatore et al. [2010],which was originally intended to be used on CRISM data.This updated parameter takes spectral slope into account,which more accurately measures the 1 mm band depth. Wereplaced the wavelengths in the CRISM parameter with anaverage of three OMEGA wavelengths centered at theOMEGA wavelength closest to the CRISM wavelength usedin Salvatore et al. [2010]. The equation for calculatingOLINDEX2 for OMEGA data, with the proper OMEGAwavelengths in nanometers, is as follows:

RC1055� R1055ð Þ=RC1055ð Þ � 0:1ð Þþð RC1213� R1213ð Þ= RC1213ð Þ � 0:1ð Þþ RC1328� R1328ð Þ=RC1328ð Þ � 0:4ð Þþ RC1472� R1472ð Þ=RC1472ð Þ � 0:4ð Þ

(1)

where RC is the reflectance value at the expected center ofthe continuum and R is the reflectance value at the wave-length specified. The weighting of each part of the parameteris the same as from Salvatore et al. [2010].[21] Detection of hydrated minerals are made primarily on

the basis of fundamental bending and stretching vibrationalabsorption bands, particularly near 1.4 and 1.9 mm wherethe 1.4 mm band is due to the structural OH stretching over-tone [Clark et al., 1990; Bishop et al., 1994, 2002] and the1.9 mm band is due to a combination tone of the bending

ba

Figure 4. (a) OMEGA mosaic using the OLINDEX2 parameter of Salvatore et al. [2010], adapted to theOMEGA wavelengths, is overlain on an HRSC mosaic. Numbers indicate where the spectra in Figure 4bwere extracted from the OMEGA data. The white box represents the area shown in Figure 5. (b) Spectralplot of the plains material clearly showing the presence of olivine absorption features. Calibrated spectraare shown on the top of the plot, with y axis values on the left and ratio spectra on the bottom of the plotwith the y axis to the right. Shades of blue and green represent numerator and denominator spectra,respectively, used to make the ratios.

Figure 5. HRSC image H0923_0000 covering part of theplains unit with a strong OLINDEX2, outlined by the boxin Figure 4. Note the smooth texture of the infill material,which retains small craters well.

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and stretching vibrations of the H2O molecule. The positionsof these bands and other diagnostic bands in the 2.1–2.5 mmrange shift, depending on the cation present in the structure.For example, absorptions near 2.2 mm are typically dueto Al-OH vibrations and 2.28–2.35 mm due to Fe/Mg-OHvibrations [Bishop et al., 2002, 2008].[22] All library spectra of igneous minerals and rocks

used in this study are shown in Figure 9. The olivine is

sample KI3189, a Fo60 sample at <60 mm particle size,taken from the USGS spectral library [Clark et al.,2007]. The pyroxenes, clinopyroxene/diopside C1PP69 ~ 1and orthopyroxene/enstatite C2PE30 ~ 2, as well as theHawaiian basalt sample CCBA01 ~ 2 were taken fromthe CRISM spectral library [Murchie et al., 2007]. Themixed layer clay library spectrum used is from Millikenand Bish [2010].

Figure 6. HiRISE image ESP_013216_1870 detailing the texture and geomorphology of the deposit in thefloor of Ares Vallis where spectral detections were made. (a) Details two sets of polygons: large, irregular,crudely hexagonal polygons, and smaller tetragonal polygons. (b) The smaller set of polygons near the ejectaof Taytay Crater exhibit a thermal contraction relationship as increasing polygon size with depth. Thisrelationship is indicated by the arrows. (c) Cone-like feature, as seen within the polygonally fractured deposit.The box outlines the area shown in part d. (d) Up-close image of the cone-like feature in Figure 6c shows thatthe feature is degraded. Please refer to the online color version of this figure for more detail.

Table 2. IDs for Imagery Used, Listed by Figure Numbera

Sensor Image ID Center Pixel Location Number of Pixels

Figures 3OMEGA ORB0401_3 (X: 100 Y: 553); (X: 126 Y: 485) 3 � 3

ORB4460_4 (X: 58 Y: 667); (X: 20 Y: 408) 3 � 3ORB4471_4 (X: 42 Y: 471); (X: 33 Y: 400) 3 � 3

(X: 37 Y: 408); (X: 33 Y: 400) 3 � 3(X: 20 Y: 387); (X: 33 Y: 400) 3 � 3

Figure 5CRISM HRL00005B77 (X: 319 Y: 130); (X: 319 Y: 636) 5 � 5

(X: 29 Y: 149); (X: 29 Y: 5) 5 � 5

Figure 8CRISM FRT00012CED (X: 426 Y: 216); (X: 426 Y: 342) 5 � 5

aSpectral data used are referenced by their center pixel locations and the number of pixels averaged is provided. Pixel loca-tions on each line refer to numerator and denominator spectra used in the spectral ratios, respectively of the unprojected images.

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4. Results

4.1. Noachian Plains Material

[23] The Noachian-aged plains to the east of Ares Vallisconsist two separate units, as mapped by Tanaka et al.[2005]: Nn and Nl (Figures 1b and 2). The older of the twounits (early to middle Noachian), Nl, is more heavily crateredand is described as reworked sedimentary and volcanicmaterial with the possibility of containing primordial crust.The younger of the two units, Nn, is middle to late Noachianin age and is described as reworked volcanic and sedimentarymaterial, including various types of resurfacing. This unit fillsin and breaches the rims of larger craters and leaves theimpression of ghost craters (Figure 2). In addition, this unitretains small craters quite well (Figure 5), indicating an in-durated or relatively competent material. Some of theplains, regardless of being assigned to unit Nn or Nl, exhibitday thermal inertia values ~400–625 Jm�2 K�1 s�1/2 andnight thermal inertia values ~400–815 Jm�2 K�1 s�1/2, asmapped using thermal emission spectrometer (TES) databy Putzig and Mellon [2007]. The day and night thermalinertia value differences on a per-pixel basis are likely afunction of vertical heterogeneity on a decimeter scale[Putzig and Mellon, 2007] but generally indicate surfacesthat can be combined with layers of rocky and nonrockymaterial. Although the modeling by Putzig and Mellon[2007] finds vertical heterogeneity exists almost everywhereon Mars, we cannot determine the number of layers involved,

layer thickness, or whether the rockier layer is exposed at thesurface or beneath the surface.[24] VNIR data from OMEGA reveal that, in both of the

Nn and Nl units, olivine dominates the spectral signature(Figures 4 and 8). Taytay crater samples a portion of thecrater-filling unit Nn. Taytay crater is an approximately18 km diameter crater located on the southern rim of a larger,~40 km diameter unnamed crater (Figures 2 and 8). Theunnamed crater has been infilled by the younger of the twoNoachian units (Nn). Taytay crater possesses an arcuateridge on the wall of its northern side which gives rise toolivine-bearing signatures in both CRISM and OMEGA data(Figures 4 and 8) [Wilson and Mustard, 2010]. Because thestrongest olivine signatures originate from the ejecta and theridge on the northern interior part of Taytay, it is clear thatTaytay samples the infilling material of the larger crater. Asmall area of hydrated material, detected in the central peakand near the southern portion of the crater, also shown inFigure 8, indicates that Taytay also excavates complexNoachian basement rocks [Wilson and Mustard, 2010]. Fur-thermore, we used the scheme of Grieve and Pilkington[1996] to measure the maximum stratigraphic uplift (SU) forterrestrial cases, which has also been applied as a viable wayof measuring the depth of material exposed in central peaksin Martian craters [e.g., Michalski and Niles, 2010; Caudillet al., 2011]. From the relationship,

SU ¼ 0:086D1:03 (2)

using an average final rim-to-rim crater diameter (D) of18.2 km, we find that the maximum stratigraphic uplift (SU)in Taytay Crater is approximately 1,700m; this indicates thatthe hydrated material is excavated from around this depth,thereby constraining the thickness of the fill unit being sam-pled in the larger, unnamed crater to be less than ~1,700m.Since crater degradation serves to decrease the apparent finalrim to rim diameter, this estimate should be considered as alower limit to the upper bound of the fill thickness.

4.2. Ares Vallis Floor Material

[25] The floor of Ares Vallis, just to the east of AramChaos, exhibits quite a different texture than that of theplains units. The floor of the channel in this region isrelatively flat, and its top-most surface is dark, appearingto embay preexisting fluvially carved features, as shown inFigure 6. The relatively darker tone of this surface showssimilarities to the widespread mafic cap recognized else-where on Mars [e.g., Noe Dobrea et al., 2010; Michalskiand Noe Dobrea 2007; Ehlmann et al., 2009] where themorphology of the mafic cap is a relatively thin obscuringlayer that shows weak pyroxene absorptions in CRISMVIS/NIR data. The layer is often so thin that the underlyingmorphology is readily apparent, but compositional contactsare obscured. The flat floor unit exhibits a polygonal texturethat is unlike the rest of the channel floor. The olivine-bearing material is observed on lighter toned material thatis, in places, relatively high standing. These may be lowmounds formed during the Ares Valles outflow event andlater embayed or represent small-scale deformation follow-ing emplacement of the floor material. Removal of a thinmafic cap would represent small erosional windows expos-ing a lighter-toned material (as seen in HiRISE and CTX

b

c

Figure 7. CTX mosaic using images B05_011515_1867,B11_013994_1870, and P13_006241_1873. (a) The unit inthe floor clearly embays the previously carved fluvialfeature, which is outlined in black as well as the walls of thechannel. (b) Additional outcrops of the light-toned materialappear farther up the channel within the material that embaysthe feature in Figure 7a. (c) Ridges that could possibly berelated to lava flow fronts are present in the unit where spectraldetections are made.

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imagery). These lighter-toned outcrops (Figures 3 and 6)occur in association with small mounds that can be seen inHiRISE anaglyph imagery (see Figure S1 in the supportinginformation). The mounds are much smoother in a topo-graphic sense than the other fluvial features exposed in thefloor of the channel (e.g., compare Figure 6 and Figure S1).[26] It has been noted by Rogers et al. [2005] that, in

addition to the light-toned regions on the Ares Valles floor,there are cliff-forming units standing above the valley floorthat are olivine bearing in THEMIS data [Rogers et al.,2005, Figures 1 and 2]. These outcrops and units

unfortunately have not been observed by CRISM and aretoo small to be resolved by available OMEGA data.[27] The exposed polygons range in diameter from <1 m

to a few tens of meters and increase in size from the edgesof the channel toward its center; they also increase in sizeas the depth of erosion increases (Figure 6b). Polygon shapeand joint types are consistent with those described by Aydinand DeGraff [1988] as a tetragonal pattern with T and curvedT joints, respectively, which are features consistent withthermal contraction. Hexagonal patterns are not observed inthis unit. The fractures that define the polygons are more

Figure 8. (a) CRISM image HRL00005B77 using the wavelengths 2.38, 1.8, and 1.15mm as red, green,and blue, respectively. Stars indicate locations of spectra used on the top spectral plot, while pointsindicate locations of spectra used on the bottom spectral plot. Black and white symbols denote the targetspectrum and neutral spectrum, respectively. (b) CTX mosaic over Taytay Crater, showing the location ofthe topographic profile shown to the right. Please refer to the online color version of this figure for more detail.

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pronounced in the light-toned material, although the fracturesare apparent in the dark-toned material that superposes thelight-toned unit here, giving the appearance of a thin, semi-transparent cover that overlies it close to the edge of thepresent-day erosional scarp. This cover is thinner in someareas than others. Superposed on the smaller polygons arelarger, irregular cracks, and where erosional windows do notexist, the surface is riddled with small pits and/or craters(Figure 6a).[28] Furthermore, one of the small mounds within the

light-toned, polygonally fractured material appears to becone-like. This feature exhibits a breached crater or pit onthe side of its peak measuring ~42m in diameter and hasan average basal diameter of ~220m; this yields a crater/base ratio of ~0.19 (Figure 6c), which is much smaller thanother identified conical features on Mars [e.g., Meresse et al.,2008, Table 1, and references therein].[29] Ridge-like features can also be identified in the floor

of Ares Vallis (Figure 7c). Although these ridges continu-ously crosscut the entire channel, the topographic relief (onthe order of up to 10 m in the MOLA gridded topography)of the ridges is consistent with the floor material as beingextremely competent, enough so to preserve small-scaletopography coinciding with running water. Additionally, thethermal inertia values for day (~550–900 Jm�2K�1 s�1/2)and night (~500–1000 Jm�2K�1 s�1/2) [Putzig and Mellon,2007] indicate that rocky material is dominating the thermalcharacteristics in the floor of Ares Vallis where the aforemen-tioned observations were made.[30] For well-exposed light-toned outcrops on the floor,

CRISM spectral data (where available) bear signatures ofolivine and pyroxene with absorptions near 1 and 2 mm(Figure 3). The area where the unit is not eroded seems to

be mafic in composition, but the spectral signatures are notnearly as strong as those present from the freshly exposedsurfaces. This is similar to the character of the mafic capelsewhere on Mars [Ehlmann et al., 2009; Michalski andNoe Dobrea, 2007; Noe Dobrea et al., 2010]. The bandcenter of the broad 2 mm pyroxene absorption is approxi-mately 2.13 mm. Because we can expect exsolved pyroxenesto act as intimate mixtures of two pyroxenes and becauseband center does not vary with absolute modal abundances[Sunshine and Pieters, 1993], we report the pyroxene compo-sition relative to the amount of HCP versus LCP. Accordingly,by inspecting the 2 mm band center, we find that this materialcomposing the light-toned units is indicative of a pyroxenethat is composed of an approximately 50:50 mixture of HCPand LCP.

5. Discussion

5.1. Origin of the Ares Vallis Floor Material

[31] There are a number of possibilities that could lead to thepresence of an olivine-pyroxene basaltic signature on thefloor of Ares Vallis including prechannel emplacement orpostchannel emplacement of the igneousmaterial. Other ideas,such as formation of outflow channels by lava, have beenproposed [e.g., Leverington, 2011]; however, specific exam-ples for Ares Vallis have not been documented extensivelyenough to support this conclusion over a water-carvingscenario, and this hypothesis is not considered here.5.1.1. Prechannel Formation of Ares Vallis FloorMaterial?[32] We could attribute the flat character of the Ares Vallis

floor to infill by fluvially deposited sediment, aeolian material,volcanism, or a combination of these factors. The simplestway to explain the igneous signature on the floor of AresVallis is by assuming that the current depth of the channelrepresents the depth of excavation during channel formation.A detailed study compared Ares Vallis to the Lena River inSiberia and showed that the width to depth ratio and dischargerate of Ares Vallis is consistent with those of rivers inperiglacial environments [Costard et al., 2007]. However, thisdoes not address the detail of the channel floor morphologybetween streamlined islands.[33] If the surface of the floor of Ares Vallis is, indeed, a

solely erosional surface, a plausible explanation for hetero-geneity in this area can be accounted for through studies ofMartian impact basins by Schultz and Glicken [1979],Schultz and Schultz [1980], and Schultz et al. [1982]. Theauthors identify many basins on Mars that have beencovered, only to be reexpressed on the surface by endogenicprocesses and features. In particular, Schultz et al. [1982]detail that the Ares Vallis region of Mars has been affectedby two basin-forming impacts currently identified as ChryseBasin and Aram Chaos. According to this work, deep-seatedfaulting as a result of basin formation is reexpressed bychaos and outflow channels, among other features. Thiscan be seen by examining the oddly angled bends that AresVallis exhibits (Figure 1a). Schultz et al. [1982] also statethat these deep-seated, preexisting listric faults can be con-duits for volcanism. The fact that igneous signatures areexpressed on the floor of the Ares Vallis between two cross-cutting wrinkle ridges (in Figure 1, the area where the Aram

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.5 1 1.5 2 2.5

Clinopyroxene

Orthopyroxene

Basalt (scaled x 2.5)

Olivine

Corrensite

Lab

ora

tory

Ref

lect

ance

Wavelength (µm)

Figure 9. All library spectra are plotted for comparison tothe spectra shown in Figures 3, 4, and 8.

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and Chryse basin rings overlap), at a proper depth to find thelistric faults (~2 km), in relation to the surrounding crustcould be consistent with magma that ascended through thefaults formed due to the Aram impact, intruded theprechannel crust, and was exposed by channel formation.[34] Edwards et al. [2008] imply that olivine-rich, high

thermal inertia deposits in this area may be linked to thosein Valles Marineris at similar elevation by global floodingof lava early in Mars history. This scenario presents somechallenges in explaining our observations. Here we havedocumented that the older plains to the east of Ares Vallisare more olivine-rich than that of the floor of Ares Vallis.The differing mineral assemblage between the plainsmaterials and the floor of Ares Vallis, as well as the age ofthe channel floor and the fact that the unit appears to fillthe channel, may point to a time-related compositionalchange in igneous character. One might expect a cataractof sorts to be formed if water flowed over either the contactof two different materials with differing competency or thecompetent material itself (cf. cataracts identified by Warneret al. [2010] in a tributary to Ares Vallis). No such featurehas been identified in the main channel in this study.5.1.2. Postchannel Formation of Ares Vallis FloorMaterial[35] The floor of Ares Vallis may be a volcanic deposit

emplaced postchannel formation. The high thermal inertia ofthe area in which spectral detections are made relative to theremainder of the channel implies that there is a relativelycompetent/rocky material present. Furthermore, althoughpolygons alone are not definitive evidence of a volcanicdeposit, the thermal inertia and spectral signatures are stronglyconsistent with volcanic materials. The ridges that crosscutAres Vallis may not be related to the volcanic unit on the floorof the channel; however, they do indicate that in order for suchsmall relief to be preserved, the material must be very compe-tent. Although the light-toned outcrops that show strongolivine-pyroxene signatures occur on low, local topographicrises (Figure S1), we see embayment relationships along theedges of the channel, as well as at the contact with the previ-ously carved fluvial features. The local topographic highsmay have had a thin obscuring, mafic cap removed to permitobservation of the underlying mafic signatures. This embay-ment relationship can be clearly seen in Figures 7a and 7b. Itis not clear whether all light-toned outcrops (e.g., those shownin Figure 7 as opposed to those shown in Figure 6) occur onlow rises, but these can be easily explainable. The low riseshave gradual slopes and topography on the order of 5–6 m,as observed in MOLA gridded elevation data; a lava flowdraped over previously present topography could formundulating rises as seen here through deflation. It is possiblethat postemplacement structural deformation or settlingof the lava flow could have formed these mounds. Thepostemplacement deformation hypothesis is not favoredbecause a significant structural orientation trend does not existfor other streamlined features in the channel; in addition, otherstreamlined features that are much higher in elevation areembayed by the unit that appears to contain the olivine-bearing unit. Therefore, it would appear that the lava fillreached some intermediate height that covers smaller floorfeatures, but did not reach the top of the taller streamlinedfeatures. Rogers et al. [2005] noted that cliff-forming unitsat the base of some streamlined islands showed THEMIS

signatures that they are olivine-bearing. Unfortunately, noCRISM or OMEGA data exist at a resolution to confirm thatobservation.[36] Some researchers [e.g., Robinson et al., 1996] have

suggested that the mouth of Ares Vallis has been filled withHesperian volcanic material. It is possible that a volcanic floworiginating from the Chryse region could have filled a largeportion of Ares Vallis; however, according to current topogra-phy, the area where spectral detections are made is coincidentwith a surface that would have required the lava to flow uphillfrom the Ares Vallis mouth (cf. Figure 10). Also, VNIRCRISM data from the Acidalia/Chryse region [Salvatore,2010] show that the compositions of the pyroxenes there are

Figure 10. MOLA topography data showing the (a) longi-tudinal profile of Ares Vallis and (b) various profiles takenperpendicular to the channel labeled by segment name. Notethe flat nature of the channel floor in the perpendicularprofiles. Please refer to the online color version of this figurefor more detail.

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more HCP-rich than those identified on the floor of AresVallis. Therefore, this hypothesis is not preferred. Sincesource vents for Hesperian volcanism are rarely identifiable,it is likely that the source for this material may be obscured.

5.2. Importance of the Noachian-Aged Plains Material

[37] If it is possible to document Noachian lava flows origneous outcrops that all have the same general spectralcharacter, we may be able to constrain a more global processfor extensive production of olivine-rich igneous materials.Earth’s surface rarely exhibits true signatures of meltsdirectly from the mantle due to crustal assimilation andextensive fractional crystallization, but work by McSweenet al. [2006] shows that Mars’ surface may be dominatedby primary magmas and thermal modeling by Baratouxet al. [2013] implies that we can assume that Hesperian-aged volcanism on Mars should be sourced directly fromthe mantle. Following that logic, if Hesperian-aged terrainsexperienced volcanism that was sourced directly from themantle, it is likely that Noachian terrains did also. Becausethe olivine phase field expands as pressure decreases, wecan generally assume that all melts produced by mantlemelting are olivine saturated and will crystallize olivine first,regardless of mantle temperature. Dunite channels in perido-tite on Earth [e.g., Kelemen et al., 1995] give clear evidencefor this process. For a mantle that is hotter in comparisonto one that is colder, melts would have to fractionallycrystallize more olivine to get to pyroxene and plagioclasesaturation; in addition, it is plausible that the lithospheremay be thinner for a hotter mantle as opposed to a coolermantle. If it can be demonstrated that very olivine-richvolcanic material is increasingly present in Noachian igneousterrains, a scenario where either a hotter mantle and/or athinner lithosphere in the Noachian as compared to the Hespe-rian is more likely to be the process that formed the trend involcanism from more LCP enriched to more HCP enriched,as noted by Mustard et al. [2005]. If this is indeed the case,then confirming the lack of plagioclase in the unit studiedherein, as noted by [Rogers et al., 2005], would be a compli-ment to the pyroxene distribution seen through time byMustard et al [2005]. In addition, being able to discern thecomposition of the olivines we detect can be a third confirma-tion that the Noachian volcanism sourced from a hotter mantle(i.e., melts from a hotter mantle should be more Fo-rich) thanthat of Hesperian volcanism.

6. Conclusions

[38] In this study, we analyze the morphology, stratigraphy,and VNIR spectral properties of volcanic deposits from theNoachian and Hesperian time periods in the Ares Vallisregion. We interpret these data to show that at least some ofthe most recent volcanic deposits on the floor of Ares Valliswere emplaced in the Hesperian. We interpret the diagnosticVNIR crystal field absorptions, observed in the reflectancespectra and exposed in light-toned mounds that may beerosional windows from beneath a thin cover, as olivine-richbasalts, containing a compliment of pyroxenes with anapproximately 50:50 mixture of LCP and HCP. Thiscomposition is more typical of other Hesperian basalts onMars which contain LCP, HCP, and olivine, as opposed toNoachian igneous terrains which contain much less HCP.

Alternatively, these light-toned outcrops could be exten-sions of the olivine-bearing, cliff-forming units describedby Rogers et al. [2005], in which case they are units thatpredate the formation of Ares Valles. We feel the availabledata favor the former interpretation, but additional high-resolution compositional data would be required tocompletely resolve.[39] The Noachian-aged plains on the plateaus to the east

of Ares Vallis have also likely been volcanically resurfacedand exhibit distinct olivine-rich compositions in depositsthat fill craters and low-lying areas—a signature which isfundamentally different than that of the floor of Ares Vallis.We interpret the mineralogy to reflect an olivine basaltcomposition which is not typical of Hesperian lavas in thatit shows no definitive spectral evidence for the presence ofpyroxene in the VNIR.[40] If the units exposed on the floor of Ares Vallis are

Hesperian-aged volcanics, similar to the large volcanic unitsto the west and north of the mouth of Ares Vallis, theseanalyses capture an evolution in the mineral assemblagefrom olivine-rich Noachian volcanics to HCP-plagioclasedominated Hesperian volcanics. This difference conformsto the trend observed in VNIR data with LCP as thedominant mafic mineral in older terrains and HCP is a moreprominent component in younger terrains. Better constraintson the ages of the units in this region and additionalinstances of compositional-temporal differences of volcanicterrains across the Noachian-Hesperian boundary will beimportant to test the hypotheses of a significant change inMar-tian volcanism at this time. Could the pyroxene compositionof the basalt in the floor of Ares Vallis represent a “missinglink” to the LCP to HCP transition over time? These twoobservational conclusions, combined with similar observa-tions across the globe, imply that some igneous compositionalchange has occurred between the Noachian andHesperian, butthis change is only now being assessed. The change in pyrox-ene type (LCP to HCP) is corroborated through modeling byBaratoux et al. [2013]. More observations and analysis areneeded to evaluate these proposed trends and especially tomake inferences for mantle properties.

[41] Acknowledgments. We are very grateful for the dedicated workof the NASA MRO project team and the excellent job done by theCRISM Science Operations Center (SOC). This work was supported byNASA through NASA MDAP contract NNX07AV41G to JFM and sub-contract JHAP852950 to the Johns Hopkins University Applied PhysicsLaboratory.

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